On February 5, 2019, The Galaxy reported that if advanced, space-faring alien life exist anywhere in the Milky Way, odds are they will live in the Galactic Habitable Zone (GHZ). And if they want to explore the Milky Way searching for other life-bearing planets, similar to Earth, they would only have to explore the GHZ, not the whole of the Galaxy.

Astronomers have modeled the evolution of the Milky Way Galaxy over the past two decades to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets similar to Earth, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae or gamma ray bursts.

In 2018, within a field of view that represents 1/400th of the Milky Way but in the heart of the GHZ, researchers scanned 14 worlds From the Kepler Mission for “technosignatures”, evidence of advanced civilizations. Between Kepler‘s first and second mission (K2), a total of 5,118 candidates and 2,538 confirmed exoplanets have been discovered within our galaxy alone. As of February 1st, 2018, a grand total of 3,728 exoplanets have been confirmed in 2,794 star systems, with 622 having more than one planet all within the GHZ.

While the exact boundaries of the GHZ are not clear, the inner regions of the Milky Way have plenty of metals but are hazardous for life, while the outer regions of the thin disc are safer, but metal-poor and less likely to contain Earth-like planets.

In between, there is the GHZ, with our Solar System sitting near its center. We live in a particularly favorable location in the Milky Way for life. We also live at a special moment in the Milky Way for life. Until about 5 billion years ago, even apart from the shortage of metals needed for like’s origin, the activity of star birth and the supernovas would have made life hazardous.

It has taken almost all of 5 billion years for intelligence to evolve on Earth, and if that is typical, we may be one of the first intelligent civilizations in our Galaxy. There is indeed something that appears to be unique about our place the Milky Way, in both time and space.

“While most of the stars that Kepler observed are typically thousands of light years away from the Sun, Kepler observed a large enough sample of stars that we can perform a rigorous statistical analysis to estimate of the rate of Earth-size planets in the habitable zone of nearby sun-like stars.” said Danley Hsu at Penn State University.

A new study from Penn State University provides the most accurate estimate of the frequency that planets that are similar to Earth in size and in distance from their host star occur around stars similar to our Sun. Knowing the rate that these potentially habitable planets occur will be important for designing future astronomical missions to characterize nearby rocky planets around sun-like stars that could support life. A paper describing the model appears August 14, 2019 in The Astronomical Journal.

Thousands of planets have been discovered by NASA’s Kepler space telescope. Kepler, which was launched in 2009 and retired by NASA in 2018 when it exhausted its fuel supply, observed hundreds of thousands of stars and identified planets outside of our solar system–exoplanets–by documenting transit events. Transits events occur when a planet’s orbit passes between its star and the telescope, blocking some of the star’s light so that it appears to dim. By measuring the amount of dimming and the duration between transits and using information about the star’s properties astronomers characterize the size of the planet and the distance between the planet and its host star.

“Kepler discovered planets with a wide variety of sizes, compositions and orbits,” said Eric B. Ford, professor of astronomy and astrophysics at Penn State and one of the leaders of the research team. “We want to use those discoveries to improve our understanding of planet formation and to plan future missions to search for planets that might be habitable. However, simply counting exoplanets of a given size or orbital distance is misleading, since it’s much harder to find small planets far from their star than to find large planets close to their star.”

To overcome that hurdle, the researchers designed a new method to infer the occurrence rate of planets across a wide range of sizes and orbital distances. The new model simulates ‘universes’ of stars and planets and then ‘observes’ these simulated universes to determine how many of the planets would have been discovered by Kepler in each `universe.’

“We used the final catalog of planets identified by Kepler and improved star properties from the European Space Agency’s Gaia spacecraft to build our simulations,” said Danley Hsu, a graduate student at Penn State and the first author of the paper. “By comparing the results to the planets cataloged by Kepler, we characterized the rate of planets per star and how that depends on planet size and orbital distance. Our novel approach allowed the team to account for several effects that have not been included in previous studies.”

The results of this study are particularly relevant for planning future space missions to characterize potentially Earth-like planets. While the Kepler mission discovered thousands of small planets, most are so far away that it is difficult for astronomers to learn details about their composition and atmospheres.

“Scientists are particularly interested in searching for biomarkers–molecules indicative of life–in the atmospheres of roughly Earth-size planets that orbit in the ‘habitable-zone’ of Sun-like stars,” said Ford. “The habitable zone is a range of orbital distances at which the planets could support liquid water on their surfaces. Searching for evidence of life on Earth-size planets in the habitable zone of sun-like stars will require a large new space mission.”

How large that mission needs to be will depend on the abundance of Earth-size planets. NASA and the National Academies of Science are currently exploring mission concepts that differ substantially in size and their capabilities. If Earth-size planets are rare, then the nearest Earth-like planets are farther away and a large, ambitious mission will be required to search for evidence of life on potentially Earth-like planets. On the other hand, if Earth-size planets are common, then there will be Earth-size exoplanets orbiting stars that are close to the sun and a relatively small observatory may be able to study their atmospheres.

Based on their simulations, the researchers estimate that planets very close to Earth in size, from three-quarters to one-and-a-half times the size of earth, with orbital periods ranging from 237 to 500 days, occur around approximately one in four stars. Importantly, their model quantifies the uncertainty in that estimate. They recommend that future planet-finding missions plan for a true rate that ranges from as low about one planet for every 33 stars to as high as nearly one planet for every two stars.

“Knowing how often we should expect to find planets of a given size and orbital period is extremely helpful for optimize surveys for exoplanets and the design of upcoming space missions to maximize their chance of success,” said Ford. “Penn State is a leader in brining state-of-the-art statistical and computational methods to the analysis of astronomical observations to address these sorts of questions. Our Institute for CyberScience (ICS) and Center for Astrostatistics (CASt) provide infrastructure and support that makes these types of projects possible.”

The Daily Galaxy via The Center for Exoplanets and Habitable Worlds at Penn State